U.S. Pat. No. 3,234,893 assigned to the same assignee as the present application discloses a hitch head having a pair of jaws pivotably mounted on a pair of vertical pins extending between upper and lower head plates. Torsion springs bias the jaws to open position. However, a yoke engages the jaws to hold the jaws in closed position and the yoke is biased into this closed position by a stiff spring. In closed position the jaws hold a trailer kingpin in place.
In this arrangement the jaws bear the load when the kingpin is forced toward the front or open end of the hitch head. However, the top plate opening is dimensioned such that the top plate bears the load when the kingpin engages the rear, or closed end of the hitch head. This same type of head loading is also present, for example, in application Ser. No. 009,334 filed Feb. 5, 1979.
This type of hitch head loading is believed to be the case in other hitch head arrangements such as that disclosed in U.S. Pat. No. 3,632,145.
However, with this type of hitch head loading, the top plate is subject to wear due to the kingpin loads which it must absorb in use.
When the hitch top plate has worn sufficiently that there is danger of kingpin escape, it has been common practice to apply weld metal to the top plate wear surface to return the wear surface to proper dimensions to maintain the kingpin in place. This is a time-consuming operation which generally requires that the flat car be taken out of revenue service for the welding operation. This loss of flat car revenue service time is expensive. See ACF Maintenance Manual, Model 5 Hitch, October 1975, Main and Clark Streets, St. Charles, Mo. 63301. Copy in application file.
It has also been common practice to weld an insert of hard, wear resistant material to the head or to harden the head itself or to locally harden a portion of the head.
In application Ser. Nos. 914,313 filed June 12, 1978 and 949,721 filed Oct. 10, 1978, assigned to the same assignee as the present application, it has been proposed to utilize a hardened steel replaceable insert in the top plate to overcome this wear problem.
However, to accommodate the loads applied by the kingpin to the jaws, the jaws are made of hardened steel.